(1) Field of the Invention
The present invention relates to IP (Internet Protocol) telephony systems. More particularly, the invention relates to a high functionality telephone and a call connection control method capable of selectively connecting a call originated by entering a telephone number for a Public Switched Telephone Network (PSTN) subscriber through an IP telephone network.
(2) Description of the Related Art
With the development of IP technology, Internet service providers started IP telephony services as one service content. Due to the cost effectiveness of IP telephony (Voice over IP: VoIP), lower telephone charges than conventional PSTN telephony, there is a steep rise in the number of IP telephony users. As the IP telephony comes into popular use, a telephone numbering scheme (“050” numbers) has been fixed for the IP telephony. In the current situation, new IP network telephone numbers are assigned to the telephones of IP telephony service subscribers in addition to the conventional PSTN telephone numbers.
As one of prior art approaches for making a telephone usable in two telephony systems based on different telephone numbering schemes, for example, in Japanese Published Unexamined Patent Publication No. Hei 11-257171 (Japanese Patent Document 1), a CATV telephony system which made a telephone applicable to both a public telephone network and a CATV telephone network has been proposed.
According to the Japanese Patent Document 1, when a CATV telephone service subscriber originates a call, the call is routed to a switch on the CATV telephone network if the destination telephone number is any number other than special numbers such as “100,” “199,” “0120,” and international telephone numbers. The switch on the CATV telephone network has a telephone number translation database for translating public telephone network subscriber numbers (hereinafter referred to as PSTN telephone numbers) to CATV subscriber telephone numbers. If the destination telephone number is a PSTN telephone number, the switch searches the telephone number translation database for a CATV telephone number corresponding to the above destination PSTN telephone number. If the CATV telephone number is found, the switch performs call connection on the CATV telephone network, using the CATV telephone number. If not, the switch routes the call to the PSTN network.
While PSTN telephone numbers are made open to the public by a telephone directory or the like, IP network telephone numbers are not known by a caller, unless an IP telephone user makes its number known to the public beforehand. Thus, IP telephone service subscribers cannot help calling each other using conventional PSTN telephone numbers, unless knowing the IP network telephone number of a called party. In this situation, a problem is presented that callers cannot enjoy the benefit of IP telephony, less telephone charges than the PSTN telephony. It is therefore desirable for IP telephone service subscribers that a call originated by entering a PSTN telephone number is automatically connected through the IP network to the called party if the destination telephone is also an IP telephone service subscriber. In order to satisfy this need, it is a challenge how to associate an entered PSTN telephone number with destination telephone's IP address information.
In an IP network that applied a gatekeeper, as a call control server (call agent), which executes, for example, a session control protocol standardized by ITU-T recommendation H.323, call connection between a caller telephone and a destination IP telephone is established by notifying from an IP telephone to the gatekeeper an IP network telephone number of the destination, and by returning destination address information such as an IP address corresponding to the above IP network telephone number from the gatekeeper to the caller telephone.
Unlike local networks such as the CATV network, the IP network is a large-scale network providing a wide range of services. Thus, it is desirable to establish call connection on the IP network for a call originated by entering a PSTN telephone number by making effective use of existing network resources and without altering the standardized call connection protocol run by the above gatekeeper.
In the telephony system disclosed in the Japanese Patent Document 1, IP network connection for special numbers is not taken into consideration, that is, a caller terminal which originates a call by entering a special number is connected to the PSTN. As for calls originated by entering a PSTN number other than special numbers, the switch on the CATV network always executes database search per call. When this system is assumed to be applied to the large-scale IP network, a vast database is required and this poses a problem of a low speed of call connection processing.